Modems were developed to allow computers to exchange information over a network of telephone lines. To process information, a computer reduces data to a digital format of 1's and 0's, representing the two values by either the presence or absence of an electronic signal. The modem, which is short for modulation/demodulation, converts this digital representation to sounds which, in turn, are coded by the telephone lines as electrical signals. In this modulated or analog format, the digital 1's and 0's are represented by different frequencies within a defined bandwidth. At the receiving end of the transmission, another modem converts the signal from frequency form back to digital form so that the data can be accepted and processed by the receiving computer.
The key performance parameter for a modem is its data transfer rate, which is usually measured by baud rate, or the number of bits per second the modem can reliably generate and receive. Baud rates of 28,800 and 56,600 are now commonplace in PC communications.
On land, the medium between modems is the benign environment of a shielded wire or the sharply defined path of a microwave transmission. In these environments, it is relatively easy to achieve fast and reliable transmission of large amounts of data. Without much interference, the discrete signals can be sent out in very close proximity and still be properly understood at the receiving end. And, as signals begin to fade over distance, network facilities recondition the signals so that they arrive in a clear, unambiguous form.
Unfortunately, in many underwater applications, a wire connection with submerged instrumentation is either prohibitively expensive or not feasible. The solution is to use the water itself as the medium for the transmission of acoustic signals. However, this solution presents several problems. First, sound travels through water at a much slower speed—approximately 1,500 meters per second—compared to electrical transmissions on a phone line, which travel at the speed of light.
Secondly, the water is an open channel into which the acoustic signal is broadcast. Even when the transmission is a narrow beam aimed at its target, the sound wave fans out and generates echoes which arrive at the target destination shortly after the original signal. These multipath echoes require additional processing as the signal is received.
The open-channel broadcast also results in the need for additional signal processing with each transmission to assure that the target, and only the target, receives the message. Finally, water can be a much more hostile environment. The signal is affected by changes in water temperature, turbulence, objects in the water and a host of other factors, including any relative motion between communication nodes.
With any wireless communications system provisions must be made to accommodate rapid relative velocity between transmitters and receivers of the system. This is especially true of underwater communications, which have relatively limited bandwidths and otherwise difficult channels. All communications signals contain components used for acquisition and alignment, where, in the broadest sense, alignment pertains to both the temporal and spectral identification of the modulated portion of the larger signal. A typical signal component is a linear frequency modulated (LFM) waveform (also known as a chirp). This is processed with a “matched filter” technique using as a filter an exact replica of the transmitted LFM. The peak of the filtered output indicates the arrival time of the signal. When relative velocity (i.e., range rate) occurs, the waveform is distorted by temporal compression or dilation, which also has the effect of compressing/dilating the spectral content of the waveform. In this case, the basic filter is no longer a good “match” for the received signal. The distortion causes a decrease in the peak filtered response, as well as loss of precision in estimation of temporal alignment. Furthermore, the level of spectral distortion of the signal is not revealed. The issue here is to develop an acquisition/synchronization subsystem which can provide acquisition of a packet and provide satisfactory alignment with the modulated message over a wide span of range rates. At the same time the acquisition must provide initial estimation of the range rate so the remainder of the signal can be corrected to enable the demodulation to proceed as if there were no motion present.
The classic method for solving this problem is to form a multi-hypothesis, maximum likelihood estimator, wherein a “bank” of filters are formed, each reflecting a different hypothesis of range rate. The number of filters used must account for the degree of spectral distortion imposed by the motion. Typically, a new filter must be used when the adjacent filter peak is reduced by 50%. The system then observes all of the filtered outputs and chooses that one with the largest peak. This “best” choice of filter then determines the range rate, which can then be used to correct the remainder of the signal for the imposed spectral distortion.
The approach just described is considered optimal under typical conditions of an additive white Gaussian noise channel. However, the computational burden is very high, and, may be prohibitive for a small, battery-powered digital signal processor (DSP).
A principal purpose of this invention is to provide an alternative transmission/acquisition signal, which is robust in the presence of range rate, and which is combined with a secondary signal to identify the range rate where the combination of the two is used for purposes of both temporal and spectral alignment.
Another purpose of this invention is to provide improved underwater acoustic modems that can conduct bi-directional communication while moving at high speeds relative to one another.
Other objects of the invention will, in part, be obvious and will, in part, appear hereinafter when the following detailed description is read in conjunction with the drawings.